The delivery of biomolecules, including lipids and therapeutic proteins, provides a promising vehicle for the treatment of many diseases and conditions, such as angiogenesis. Angiogenesis is essential for tissue development, function, maintenance, repair and regeneration. Impaired angiogenesis due to either injuries or diseases can severely impair these processes. (Carano et al. 2003 Drug Discov Today 8:980-9; Laschke et al. 2006 Tissue Eng 12:2093-104; Harris et al. 2013 Curr Pharm Des 19:3456-65; Novosel et al. 2011 Adv Drug Deliv Rev 63:300-11; Nguyen et al. 2012 Tissue Eng Part B Rev 18:363-82.) For instance, disruption of vascular network as a result of orthopedic trauma compromises the ability to vascularize bone grafts, resulting in high clinical failure rates of bone graft-mediated repair of traumatic bone defects. (Ito et al. 2005 Nat Med 11:291-7.) In pathological conditions such as diabetes, the microangiopathic complication/tissue ischemia also retards bone injury repair and graft healing as it disrupts the tightly coupled osteogenesis and angiogenesis processes. (Abaci et al. 1999 Circulation 99:2239-42; Waltenberger et al. 2001 Cardiovasc Res 49:554-60; Kanczler et al. 2008 Eur Cell Mater 15:100-14.)
Therapeutic strategies for promoting angiogenesis, particularly the formation of functional and stable vascular network, have long been sought after in scaffold-assisted tissue repair and regeneration. Angiogenesis involves a dynamic cascade of cellular and molecular events involving early-stage of lumen formation (e.g., increased blood vessel permeability, basement membrane degradation, endothelial cell (EC) migration, proliferation and further assembly into tubular structure) and later-stage of nascent EC tube stabilization and maturation (e.g., mural cells recruitment and new basement membrane deposition). (Carmeliet et al. 2011 Nature 473:298-307; Potente et al. 2011 Cell 146:873-87.) The entire angiogenesis process is tightly regulated by a dynamic balance of pro-angiogenic factors and vessel-stabilizing factors. (Jain 2003 Nat Med 9:685-93.)
Current strategies for recapitulating this process in-situ involve the delivery of angiogenic stimuli, of which angiogenic growth factor such as vascular endothelial growth factor (VEGF) is the most intensively studied. (Nguyen et al. 2012 Tissue Eng Part B Rev 18:363-82; Baiguera et al. 2013 Angiogenesis 16:1-14; Cenni et al. 2011 Acta Pharmacol Sin 32:21-30; Said et al. 2013 J Vasc Res 50:35-51; Mehta et al. 2012 Adv Drug Deliv Rev 64:1257-76; Tayalia et al. 2009 Adv Mater 21:3269-85.) VEGF is a potent angiogenesis initiator that is also known to disrupt pericyte coverage and inhibit subsequent vessel stabilization, thus the delivery of exogenous VEGF alone often results in sub-optimal neovascularization characterized with immature “leaky” vessels. (Greenberg et al. 2008 Nature 456:809-13.)
Therefore, the delivery of alternative/complementary signaling molecules promoting the formation of more extensive, stable and functional vascular network are highly desired. Phospholipid sphingosine 1-phosphate (S1P) has emerged as such a promising candidate because of its dual role as angiogenic stimulant and blood vessel stabilizer.
During the early stages of angiogenesis, S1P acts as a potent EC chemoattractant, promoting EC proliferation, migration and further assembly into tubes while S1P receptor 1 (S1P1) negatively regulates vessel sprouting to prevent excessive sprouting. (English et al. 2000 FASEB J 14:2255-65; Yatomi et al. 2000 Blood 96:3431-8; Kimura et al. 2000 Biochem J 348 Pt 1:71-6; Lee et al. 1999 Cell 99:301-12; Shoham et al. 2012 Development 139:3859-69; Gaengel et al. 2012 Dev Cell 23:587-99.) In the later stages of angiogenesis, S1P regulates vasculature remodeling and maturation by recruiting vascular smooth muscle cells (VSMC) and pericytes. (Takuwa et al. 2010 World J Biol Chem 1:298-306; Paik et al. 2004 Genes Dev 18:2392-403; Liu et al. 2000 J Clin Invest 106:951-61; Allende et al. 2003 Blood 102:3665-7.)
Studies support potential benefits of the delivery of S1P in improving the functional outcome of tissue repair. The local delivery of S1P or S1P analogue FTY 720 has been shown to enhance wound healing in diabetic rats, stimulate blood flow in ischemic limbs, and promote calvarial bone formation and allograft incorporation. (Kawanabe et al. 2007 J Dermatol Sci 48:53-60; Qi et al. 2010 Eur J Pharmacol 634:121-31; Sefcik et al. 2008 Biomaterials 29:2869-77; Petrie et al. 2010 Tissue Eng Part A 16:1801-9; Das et al. 2013 J Biomed Mater Res A: doi/10.1002/jbm.a.34779; Petrie et al. 2010 Biomaterials 31:6417-24; Huang et al. 2012 Cell Tissue Res 347:553-66.)
There are few existing biomaterials that can adequately meet the requirements of the tunable and sustained delivery of such amphiphilic molecules. Poly(lactic-co-glycolic acid (PLGA) is commonly used for S1P delivery by physical blending or microsphere fabrication (Qi et al. 2010 Eur J Pharmacol 634:121-31; Sefcik et al. 2008 Biomaterials 29:2869-77; Petrie et al. 2010 Tissue Eng Part A 16, 1801-9). The release of S1P in these materials is mainly dominated by passive S1P diffusion and polymer scaffold hydrolytic degradation, which are poorly controlled by nature. The other material attempted for S1P delivery is polyethylene glycol (PEG)-based hydrogels cross-linked by albumin (Wacker et al. 2006 Biomacromolecules 7, 1335-43). The disadvantages of the system include multi-step chemical synthesis, complicated hydrogel formulation and the requirement of preloading of drug cargo in order to achieve reasonable eluting profiles. The hydrogel itself per se does not possess intrinsic structural tunability to enable manipulation of the S1P release kinetics.
Recently a cellulose hollow fiber-based system enabling timed delivery of S1P following earlier release of VEGF was shown to result in greater recruitment of ECs and higher maturation index of formed vessels in a Matrigel plug model. (Tengood et al. 2010 Biomaterials 31:7805-12.) However, this delivery system required external manual regulation, which complicates its implementation for in vivo tissue regeneration. Overall, synthetic scaffolds demonstrating significantly improved S1P loading efficiency and more tunable S1P release kinetics is still lacking.
Thus, a significant challenge for translating the S1P-based proangiogenic strategy to successful tissue repair is the lack of a tunable sustained release system enabling the optimization of its release kinetics for maximal stimulation of vessel formation and maturation. It is strongly desired that novel approaches and techniques be developed that enable controlled immobilization and delivery of biomolecules such as lipids and proteins.